DESCRIPTION: (Verbatim from the Applicant's Abstract) This is a competing continuation of NS 28721. In this upcoming 5-year project, the applicant will examine the hypothesis that Huntington's disease (HD) is caused by AMPA receptor (AMPAR)-mediated excitotoxicity. He proposes that the HD gene defect leads to selective striatal neuronal death by 1) enrichment in AMPARs and cortical inputs in striatal projection neurons and parvalbuminergic interneurons in HD. 2) A preponderance of GluR2 containing AMPARs in striatal neurons projecting to the internal pallidal segment accounts for their lesser vulnerability in HD and 3) The HD mutation decreases the numbers of group II mGluRs on corticostriatal terminals, thereby increasing cortical activation and excitotoxicity of striatal neurons. The main hypotheses are based on the following observations: a. The HD gene product is widely expressed in the brain, whereas the neurons killed in HD are localized to the striatum b. No cell-type specific proteins that interact with HD proteins are specific to vulnerable striatal neurons. c. The level of huntingtin in a given type of striatal neuron does not seem to correlate with vulnerability. d. There is suggestion that in HD transgenic mice (Bates R6/2), group II mGluRs are downregulated and that sEPSP frequency is increased. e. HD pathology can be reproduced by 3-NP and by quinolinic acid administration f. Neurons that die are rich in cortical input. g. Neurons that die are rich in AMPARs, some of which appear to be more deficient in GluR2 (the GluR2 hypothesis). From these observations, the applicant concludes "the HD mutation may render corticostriatal neurons destructive rather than render striatal neurons vulnerable." These hypotheses will be tested in 5 Specific Aims: Aim 1: Use single cell RT-PCR, immunoprecipitation and LM and EM immunolabeling to characterize the abundance and subunit composition of AMPA receptors present on HD-vulnerable and HD-resistant striatal interneuron and projection neuron types in rats. Aim 2: Characterize differences between striatal interneurons and projection neurons in AMPAR mediated synaptic responses to cortical input and in subunit dependent AMPA physiology (Ca permeability, rectification and desensitization) in rats Aim 3: Pharmacologically characterize the role of AMPARs and mGluR2/3-regulatable corticostriatal glutamate release in the selective death of striatal neurons in rats chronically administered 3NP, a model of HD. Aim 4: Use in-situ hybridization histochemistry, LM and EM immuno to determine in striatal neurons in monkey and in HD striatum whether the distribution of AMPAR subunits in HD vulnerable and HD resistant striatal neurons is consistent with their hypothesized role in selective neurodegeneration. Aim 5: Use in-situ hybridization histochemistry, LM and EM immuno to determine in a transgenic rat model of HD and in human HD specimens whether or not the HD mutation decreases mGluR2/3 in corticostriatal neurons and their intrastriatal terminals.